1. Field of the Invention
The present invention relates to a light source unit and a display device, and more specifically, to a light source unit and a display device capable of effective control of a light source comprising a plurality of light source elements with different emission frequencies.
2. Description of the Related Art
Liquid crystal display devices have become widely used as image display devices for personal computers and various types of monitors. Liquid crystal display devices generally include a liquid crystal display panel and a backlight unit mounted on the back of the panel. Liquid crystal display panels display images by controlling transmission of light. Backlight units generally include a light source and a plurality of optical components for effectively emitting the light from the light source toward the liquid crystal display panel. Cold cathode fluorescent lamps (CCFLs) and light emission diodes (LEDs) are well-known light source elements.
LEDs are increasingly employed as the light source in the backlight unit because they have better color characteristics than CCFLs. A plurality of LED chips emitting different colors of light are often used as light source elements. General techniques use three types of LED chips emitting red (R), green (G), and blue (B) light and adjust the luminance of each color LED, thereby achieving a light source unit emitting a desired color of light. The luminance of LED chips is adjusted to produce white light when the LED chips are used for a backlight unit of a liquid crystal display device. The use of LEDs emitting different colors of light requires mixing light from each LED to create a desired color of light.
LED chips are point light source elements emitting highly directional light. There are is various techniques for mixing light from LEDs. A typical method uses a light guide plate. The light emitted by each color of LED chip enters a light guide plate and travels through the plate as being diffused, thereby being mixed with each other. Though LEDs have good color characteristics, they have somewhat instable emission characteristics. Especially, the emission characteristics of LEDs change over time and temperature. More specifically, luminance decreases with time, and luminance varies and emission frequency shifts with temperature. Accordingly, the luminance or chromaticity of a light source unit varies with time or temperature.
Therefore, when using LEDs for a light source, it is necessary to use an optical sensor to detect the light from the LEDs and control the luminance of each LED. It is required that normally the luminosity curve of an optical sensor is the same as the human luminosity curve. Such a sensor, however, is hard to manufacture and hence it is difficult to obtain an optical sensor with desired properties.
Known as a typical sensor whose detection sensitivity changes with frequency is an optical sensor having a bandpass filter. Though sensors with the bandpass filter can detect a change in a luminance level within a wavelength band, it cannot accurately detect a change in spectrum (change in color) within the band. This is because the optical sensor detects the sum of all the wavelengths of light within the wavelength bandwidth of the band pass filter, not the intensity of each light with the wavelength within the bandwidth. As described earlier, LEDs have such characteristics that spectrum changes by emission wavelength shifts in accordance with temperature changes. The wavelength shifts result in changes in the chromaticity or luminance of the light source unit. The optical sensor with the bandpass filter, however, is incapable of accurately detecting the changes.
Another light source element whose emission characteristics change with temperature is an ultraviolet lamp. The ultraviolet lamp is used for a light source of a thermal printer, for example. Japanese Unexamined Patent Application Publication No. 2000-301748 describes a technique that controls the temperature of the ultraviolet lamp to improve the printing characteristics of color thermal printers. The luminance of the ultraviolet lamp is low at low bulb temperature, increases as the temperature rises, and eventually decreases when the temperature reaches a certain point. The bulb temperature is therefore required to be within a given range to maintain a constant luminance. The use of a cooling fan to cool the ultraviolet lamp allows the bulb temperature to be kept within a given temperature range.
The cooling fan may be controlled with a temperature sensor to measure a bulb temperature. Or, it may be controlled according to the luminance of the ultraviolet lamp detected by a plurality of illuminance sensors. This technique allows more accurate detection of emission characteristics of ultraviolet lamps and prevention of luminance gradient of a linear lamp. However, though this technique can control a light source emitting monochromatic light, it cannot effectively control a light source comprising a plurality of light source elements emitting different wavelengths of light such as a white light source unit comprising LEDs.